My manuscript was roughly 250 words long when we began the editing process. We added words as we went along. From what I've heard, this is not typical. But I tend to rely heavily on images to tell the story and my editor wanted the book to work well as a read-aloud for the youngest audiences. I agreed. I think we ended up at roughly 400 words. Probably only about 100 of my original words are in those final 400. I considered getting all math-geek on things and doing massive word counts, etc., but I decided to let that go and give you this equation:
250 original words times 1 bajillion words (about those 250 original words) from my editor divided by 1 gazillion words (about editor's 1 bajillion words) from me = 400 words (or thereabouts).
If you've ever looked at a picture book and thought, "Even I can muster up the strength to write 400 words", I encourage you to consider the equation above.
Still, I love the editing process. My editors keep me honest and they make my stories better.